Water is the most unique substance known to man. It defies chemical and physical logic by exhibiting behaviors that contradict those that are expected. For example, with a molecular weight of 18 g/mole, water should be a gas at room temperature. Water, however, is a liquid. Another contradiction, water in its solid state is less dense and floats on its liquid form. Why?
To understand the unique behavior of water, you must delve into its structure. Water is composed of two hydrogen atoms and one oxygen atom joined via covalent bonds. In covalent bonds, electrons are shared between the atoms that are joined. However, in the water molecule this sharing is not equal since the oxygen atom is very electronegative and exhibits a strong attraction for the electrons. The result is that a water molecule is polar. One end of the molecule exhibits a partial negative charge while the other end exhibits a partial positive charge. An analogy can be drawn between the water molecule and a magnet. Magnets are polar and oppositely charged poles attract. Water molecules show this same type of attraction.
The attraction between two polar water molecules forms a new type of bond called a hydrogen bond. One water molecule can hydrogen bond to up to four other water molecules. This added layer of structure explains the unique behaviors of water. At room temperature, hydrogen bonds are constantly being made and broken keeping water in the liquid state. When water cools during the freezing process, these hydrogen bonds lock the molecules into a set crystalline lattice. In this crystalline lattice, the water molecules are pushed farther apart than the molecules found in liquid water.
Due to the fact that this locked structure takes up more space than the liquid form of water, water expands upon freezing, the only nonmetallic substance known to do so. This expansion causes ice to be less dense than liquid water. Density is defined as the mass of a substance per unit volume. In ice, there are fewer molecules of water per unit volume so it is less dense and floats on liquid water. All other substances known to man, with the exceptions of gallium and bismuth, contract and become more dense upon freezing and sink in their liquid forms. Water's unusual behavior can only be explained by its polarity and tendency to hydrogen bond which causes expansion upon freezing. Since ice floats it can form on the surfaces of bodies of water creating a layer that provides insulation. This allows life to flourish even in the coldest climates on Earth.
Other unique properties of water can be traced back to its polarity. Water has a high surface tension. Water resists temperature change due to its high specific heat or ability to absorb heat. As a result, water helps to moderate climate here on Earth. Water exhibits cohesion and adhesion, sticking to itself and other solid surfaces. Water can creep up thin tubes and through fine pores, a property known as capillary action. Water acts as an excellent solvent dissolving more substances that any other leading to it being called the "universal" solvent. The list goes on.
Something as ordinary as water is actually extraordinary. It surrounds us daily and makes life possible on Earth. Yet, upon careful scrutiny, water's uniqueness is clear. A contradiction as simple as ice floating provides clues to water's far from mundane nature.
1) Campbell, Neil and Reese, Jane. Biology, 7th edition. Benjamin Cummings, San Francisco, 2005.